The Drain-Source ON Resistance (RDS(on)) of the conventional vertical metal oxide semiconductor field effect transistor (MOSFET) tends to become small and small. When the device is turned on, the RDS(on) is proportional to the power consumption of the device, so the power consumption of the MOSFET device can be reduced as the RDS(on) is reduced. The RDS(on) can be reduced by increasing the doping concentration. But sometimes the doping concentration cannot be increased as expected, since increasing the doping concentration may lower the breakdown voltage of the device. On the contrary, for the MOSFET device, the doping concentration cannot be reduced to increase the breakdown voltage without increasing unexpected RDS(on).
Therefore, a semiconductor device with both the increase of the breakdown voltage and the improvement of the RDS(on) is developed, which is named as a “super-junction device”. The super-junction device includes a composite buffer layer, wherein the composite buffer layer has alternating P-typed and N-typed doped areas and the doped areas have balanced valance. According to the study, the specific on-resistance (Ron, sp) of the super-junction device is 5-100 times lower than that of the conventional high voltage MOSFET.
Although such super-junction device has both features of high breakdown voltage and low specific on-resistance, the manufacturing process thereof is not easy. To optimize the function of the super-junction device, the alternating P-typed and N-typed doped areas of the composite buffer layer have to be doped with equivalent materials, respectively, to achieve the optimal balanced valence state, but such process is not easy to handle in practice. On the other hand, the practical maximum electrical field of the composite buffer layer has to be limited to about 2*105 V/cm, and the practical maximum electrical field of the super-junction device will confine the breakdown voltage thereof.
Therefore, the ion concentration in the doped area of the high voltage super-junction device has to be precisely controlled. However, in the practical manufacturing process, the doped ion concentration of the high resistance epi wafer used in the high voltage super-junction device is low, and the epi wafer with low ion concentration is easy to be interfered by other external counter ions, so that the breakdown voltage of the device may be reduced and the drain leakage current may be increased.
Please refer to FIG. 1, which is a schematic diagram showing an oxidation mask structure for manufacturing a deep trench of a super-junction device according to the prior art. As shown in FIG. 1, an oxidation mask layer 13, which is used as a mask in the etching process for forming the deep trench structure, is formed on an N-typed epitaxial layer 12 on an N+ substrate 11, wherein the oxidation mask layer 13 is formed by PE-TEOS or LP-TEOS. Since the PE-TEOS or LP-TEOS oxidation mask layer 13 is in contact with the N-typed epitaxial layer, if the oxidation mask layer 13 contains the easily diffusible ions, such as boron or aluminum ions, or the external environment contains the easily diffusible ions 14, these ions will diffuse to the N-typed epitaxial layer 12 during the subsequent high temperature process, which may cause unexpected variation of the doping concentration, resulting that the concentrations of the source, body, and drain of the MOS device are changed, and the breakdown voltage is reduced.
Therefore, by reason that in the manufacturing process of the deep trenched super-junction device, the compositions of the conventional oxidation mask layer 13 may cause the internal or the external ions to diffuse to the surface of the high resistance epitaxial layer and have the electrical property changed, it is necessary to provide an improved mask structure for the super-junction device to overcome the defects of the prior art.